1. Field of the Invention
The present invention relates to a weather-resistant crosslinked polyolefin composition. More particularly, the present invention relates to a weather-resistant crosslinked polyolefin sheet produced through a calendering process carried out by a calender machine, and to a method for making the sheet.
2. Description of Related Art
Polyolefin films commercially available in the prior art, such as polyethylene (PE) sheets or ethylene-vinyl acetate (EVA) copolymer sheets, all have defects in low heat resistance and low weatherability. On the other hand, weather-resistant plastic sheets for outdoor use are mostly made from polyvinyl chloride (PVC), when burned, which generates pollutants such as dioxins largely impacted on the environment and ecology.
To improve the performance of the polyolefin, either chemical or physical crosslinking is typically used to form covalent bonds between linear or branched polyolefin molecular chains. Once the polyolefin's structure is turned from an original linear polyolefin structure into a mesh or web-like structure, the crosslinked polyolefin is highly improved in heat resistance, wear resistance, viscous deformation, weather resistance and impact resistance.
Methods for crosslinking polyolefin in prior arts have three ways, i.e., radiation crosslinking method, peroxide crosslinking method and silane crosslinking method. The radiation crosslinking method involves the use of radiation equipment. The peroxide crosslinking method requires a peroxide crosslinking agent and must be performed at a specific temperature in order for the crosslinking reaction to take place. Compared with the radiation crosslinking method, the peroxide crosslinking method is advantageously more adaptive and produces higher-performance crosslinked products.
Nevertheless, a polyolefin if completely undergone a peroxide crosslinking reaction (hereinafter referred to as a crosslinked polyolefin) may present a problem in that it is not suitable for use of extrusion mechanism to produce a crosslinked polyolefin film or sheet (hereinafter collectively referred to as a crosslinked polyolefin sheet) having an uniform or even surface.
For example, both U.S. Pat. No. 6,187,847 and No. 6,656,986 disclose how a crosslinked polyolefin sheet can be made by an extrusion machine. During the mixing stage before extrusion, the process temperature must stay lower than the decomposition temperature of the peroxide used. Otherwise, some of the polyolefin will be crosslinked prematurely, causing scorching of the resultant crosslinked polyolefin film or aggregations of insoluble matter (known as fisheyes) which will compromise the surface quality of the crosslinked polyolefin film. However, if the process temperature in the mixing stage is too low, the crosslinked polyolefin resin may gel so poorly that a crosslinked polyolefin sheet cannot be evenly extruded.